In a normal type of time-of-flight mass spectrometer (TOFMS), a fixed amount of acceleration energy is imparted to ions derived from a sample component. The ions are introduced into a field-free flight space formed within a flight tube and made to fly in the same space. For each of the ions, the period of time required for the ion to fly a specific distance is measured, and the mass-to-charge ratio m/z of the ion is calculated based on its time of flight. Accordingly, if the flight distance is changed due to a thermal expansion of the flight tube along with a rise in the ambient temperature, the time of flight of each ion also changes, causing a discrepancy of its mass-to-charge ratio (which is hereinafter simply called the “mass discrepancy”). Therefore, various measures have conventionally been taken to avoid a mass discrepancy due to a thermal expansion of the flight tube.
Those measures can be roughly divided into two types: The first type of measure is to directly suppress the thermal expansion of the flight tube. The second type of measure is to perform data processing for correcting a mass discrepancy resulting from a thermal expansion of the flight tube while allowing the thermal expansion itself.
A specific example of the first type of measure is to create the flight tube from a material having a low coefficient of thermal expansion. Another example is to place the flight tube within a container which is temperature-controlled or is insusceptible to a change in the external temperature so as to suppress the temperature change of the flight tube even under a changing ambient temperature. For example, in a mass spectrometer described in Non Patent Literature 1, the flight tube is made of Fe—Ni36% (Inver®), which is a material having a low coefficient of thermal expansion, and this flight tube is placed within a vacuum-insulated container to suppress the thermal expansion of the flight tube and achieve a high level of mass accuracy.
On the other hand, an example of the second type of measure is to correct data obtained for a measurement target component, based on the result of a measurement of a reference sample having an exactly known mass-to-charge ratio. As is commonly known, this can be divided into an internal reference method, in which the reference sample is simultaneously subjected to the measurement with the measurement target component, and an external reference method, in which the reference sample is subjected to the measurement separately from the measurement target component. These methods can be considered as a technique in which a change in the flight distance is indirectly measured in the form of a change in the time-of-flight of an ion originating from a reference sample component.
Another example of the second type of measure is to directly measure the length of the flight tube with a laser distance meter and correct the data obtained for a measurement target component, based on the result of that measurement, as in the mass spectrometer described in Patent Literature 1.
Each of the conventional correction methods described thus far has advantages and disadvantages.
For example, materials which have low coefficients of thermal expansion, such as Fe—Ni36%, are expensive as compared to commonly used kinds of metal, such as stainless steel. Flight tubes are considerably large members. Using a material having a low coefficient of thermal expansion for such a member inevitably leads to a dramatic increase in the cost of the device. Placing a flight tube within an insulated container as in the device described in Non Patent Literature 1 also causes a dramatic increase in the cost of the device.
On the other hand, in the case of the correction using the result of a measurement of a reference sample, it is necessary to prepare the reference sample, and the analysis operator is forced to bear that burden. Furthermore, performing a measurement of a reference sample other than a measurement target component may cause the problem of a contamination in the device or a deterioration in the throughput of an analysis for the measurement target component.
In the method of directly measuring the length of the flight tube, it is necessary to measure an extremely small displacement, which is on the order of 1 ppm, for a comparatively large scale (e.g. 1 m or even larger). As described in Patent Literature 1, a laser distance meter is suitable for a high-precision measurement on such a large scale. However, laser distance meters are expensive and lead to a dramatic increase in the cost of the device.